1. Field of the Invention
The invention relates in general to a control method of recording speed in optical disc drives, and more particularly to a control method capable of effectively increasing average recording speed in optical disc drives.
2. Description of the Related Art
It is well known that modes of writing data to a CD-R or CD-RW by a recordable optical drive is generally classified into four categories, all of which are described separately as follow:
(I) Constant Linear Velocity Mode (Referred to Hereinafter as CLV Mode)
CLV mode indicates that the track length of a disc passing through a laser pickup head per time unit is constant, which means the disc is rotated in such a manner that a relative linear velocity between the disc and the laser pickup head is maintained at a constant value. Therefore, when the laser pickup head records inner parts of the disc, the rotational speed of the disc is higher than the rotational speed of the disc when the laser pickup head records outer parts of the disc. In other words, the rotational speed of the disc is determined by the locations of the laser pickup head under CLV mode, and the rotational speed of the disc is decreasing when the laser pickup head moves from the inner part to the outer part of the disc. Conventionally, an optic disc drive with 1× recording speed has a definition that the relative speed between the laser pickup head and the disc is about 1.2 m/s to 1.4 m/s, and the data transfer rate is 4.3218 MB/s. Moreover, the definition of each other optical disc drives with higher recording speeds has direct proportion with above mentioned values separately, such as 2×, 4×, 8×, 16×, 20×, and so on.
(II) Constant Angular Velocity Mode (Referred to hereinafter as CAV Mode).
CAV mode indicates that the disc is rotated in such a manner that a rotation speed of the disc is maintained at a constant value, which means difference of recording speed is between the inner part and the outer part of the disc when the rotational speed of the disc is constant. In other words, the recording speed is lower when the laser pickup head records data to the inner part of the disc than the recording speed when the laser pickup head records data to the outer part of the disc.
(III) Zoned Constant Linear Velocity Mode (Referred to hereinafter as ZCLV Mode).
ZCLV mode indicates that the recording surface of a disc is divided into several zones from its inner part to its outer part, and each zone of the recording surface is recorded at a controlled speed with CLV mode. Referring to FIG. 1, the recording speed in zone 1 is 16×, the recording speed in zone 2 where starts from the time point of T1 on the disc is 20×, and the recording speed in zone 3 where starts from the time point of T2 on the disc is 24×, the highest recording speed of the drive disc. It should be noticed here that ZCLV mode applies a super-link technology in order to connect the time point T1 and the time point T2 on the disc. The super-link technology establishes connection between time points on the disc which respectively correspond to stopping of disc writing so that the recording process can be stop temporally and continue writing. As a result, zone1 is linked to zone2, and zone2 is linked to zone3.
(IV) Partial Constant Angular Velocity Mode (Referred to hereinafter as PCAV Mode).
PCAV mode indicates that the recording speed is increasing as the movement of the laser pickup head from inner part to outer part of the disc until the recording speed reaches the highest recording speed that the disc allowed or the highest recording speed that the optical disc drive provides, and the recording speed will be maintained at a constant value that equals to the highest speed. Referring to FIG. 2, it is a schematic diagram showing the time on the disc when data is recorded with PCAV mode vs. the data transfer rate. The recording surface of the disc is divided into a CAV zone and a CLV zone by the time point of T4 on the disc. The disc is rotated at a constant velocity value by a motor before the time point of T4 on the disc so that the recording speed is increasing from inner part to outer part of the disc at the recording speed 4× to 10× until the time point of T4 on the disc. Then, the recording speed is maintained as 10× with CLV mode after the time point of T4 on the disc.
Referring to FIG. 3, it is a schematic diagram showing where data travels when data is recorded in a conventional optical disc drive. Conventionally, a host 1 sends data into a data buffer 3 in the optical disc drive when the optical disc drive starts to write data, and the current recording speed is called data transfer rate. Then, a servo system 5 catches data from the data buffer 3 according to the predetermined recording speed in the optical disc drive. Finally, a laser pickup head 7 is controlled by the servo system 5 to record data.
The state of the data buffer 3 is monitored during the recording process in the optical disc drive. However, buffer underrun easily happens when the recording speed is higher than the data transfer rate so that the recording process is failed. In order to reduce the probability of buffer underrun happening, the servo system 5 usually sends a command of decreasing the recording speed to the laser pickup head 7, so that the recording speed will slow down and the probability of buffer underrun happening will be reduced.
Referring to FIG. 4, it is a schematic diagram showing the time on the disc vs. the recording speed when writing speeds are lowered down. Take PCAV mode as an example. Firstly, the optical disc drive sets its recording speed of constant linear velocity zone to the highest recording speed that the disc allowed, such as 40×. But the servo system 5 will slow down the recording speed if the data transfer rate is low and buffer underrun happens, as shown in FIG. 4. At beginning, the recording speed of constant linear velocity zone is 40×, and then the recording speed is decreased to 32× according the command sent from the servo system 5 if the host 1 cannot attain 40× data transfer rate. Further, the recording speed is decreased to 24× if the data transfer rate is still low. However, the algorithm of decreasing recording speed damages the efficiency of data recording seriously. Although decreasing recording speed can ensure the data recorded and prevent buffer underrun happening, the recording speed of the optical disc drive is greatly affected because the host 1 cannot maintain same data transfer rate that equals to the highest recording speed that the disc allows. Analogously, during the decreasing process of the recording speed, link will exist between 40× and 32×, and between 32× and 24× so as to raise difficulty of reading data for the optical disc drive. Therefore, it is necessary to provide a method capable of effectively controlling the recording speed in the optical disc drive and keeping the recording quality of data.